assert that vocab size >= adaptive softmax cutoff (#214)

fairseq/modules/adaptive_softmax.py

@@ -22,6 +22,9 @@ class AdaptiveSoftmax(nn.Module):
 
         if vocab_size > cutoff[-1]:
             cutoff = cutoff + [vocab_size]
+        else:
+            assert vocab_size == cutoff[
+                -1], 'cannot specify cutoff smaller than vocab size'
 
         output_dim = cutoff[0] + len(cutoff) - 1